The present invention relates to the field of electronics assembly. More particularly, the present invention relates to a composition and method for soldering electronics components capable of withstanding high temperature applications.
Electronic packages provide unique capabilities in equipment and tools positioned in commercial and industrial applications. Increasingly, such electronics packages are installed in environments hostile to the survivability of electronics. For example, the formation temperature downhole in hydrocarbon producing wellbores can exceed 150 degrees C., and such high temperatures can destroy electronic control or instrumentation packages within hours. Many solder alloys and printed wiring board ("PWB") plating systems have been used for high temperature electronics applications, however such systems are limited by certain factors. Additionally, electronic packages utilizing surface mount technology ("SMT") are increasingly using electronics in plastic packages, and the long-term reliability of such packages is reduced by high temperature applications.
Various PWB plating processes have been developed for assembling electronics packages. The most common process for standard "low temperature" applications uses a tin/lead composition over bare copper. A copper-clad printed wiring board is dipped in or plated with a 60/40 solder comprising approximately sixty percent tin and forty percent lead. The shelf life before soldering and surface solderability of this process is good, however the integrity of the solder joint and PWB integrity is destroyed at elevated operating temperatures over 100 degrees C. At elevated temperatures, tin in the solder and copper on the PWB form intermetallics detrimental to long-term reliability.
The formation of intermetallics in tin/lead solders can be inhibited by the addition of a low stress nickel barrier. A 200 micro-inch layer of nickel prevents intermetallic formation without compromising solderability or shelf life. However this process is ineffective for applications over 150 degrees C. because the tin/lead solder contaminates the higher temperature solders, lowering the melting point and degrading the high temperature characteristics.
Other efforts to eliminate tin and lead from the PWB have been accomplished by plating gold over a nickel barrier. A 50 micro-inch gold layer prevents passivation of the nickel to provide reasonable shelf-life characteristics, and the gold dissolves in the solder upon assembly. Wave solder assembly processes perform acceptably, however gold can accumulate in the solder pot. This accumulation requires frequent monitoring of pot impurities and replacement of the pot contents. When the assembly is performed by hand or by solder printing/dispensing and mass reflow, a portion of the gold remains in the solder and the remainder of the gold remains on the component pads. High temperature operation forms brittle gold intermetallics compromising the mechanical and electrical characteristics of the solder joint. Although the impact of such gold intermetallics can be moderated by reducing the gold content, the time for passivation of the nickel and related unsolderability of the board is also reduced.
Another technique for incorporating the positive benefits of gold is performed by depositing approximately 10 micro-inches of electroless pure gold over 10 micro-inches of palladium over 200 micro-inches of low stress nickel. Acceptable shelf life is obtained because the palladium slows the nickel passivation and the gold slows the palladium passivation. This combination is preferable to a 50 micro-inch gold layer because the overall amounts of gold and palladium dissolved into the solder are reduced. However, this process is not conventionally used because the PWB manufacturing process for this combination is more complex and expensive than other processes.
Another technique uses pure tin over nickel and copper. This process was developed for use with high tin solders, and uses at least 200 micro-inches of pure tin deposited directly over a nickel barrier and fused to provide a low stress solderable surface. The shelf life and solderability of this combination is excellent, however this process is unsuitable for high lead solders because the mix results in a low temperature solder alloy. The affinity of tin for copper reduces the process reliability for certain applications, limiting the utility of this process.
Various solders are commonly used for high temperature electronics. References to conventional solders are found in Lea, A Scientific Guide to Surface Mount Technology, Electromechanical Publications Ltd. (UK) (1988), and in Manko, Solders and Soldering, 3.sup.rd Edition, McGraw-Hill (1992). The most common solder alloy is known as Sn63, having 63% tin with a lead balance. This solder is a binary eutectic with a 183 degrees C. melting point. Although this solder is compatible with all of the plating systems described above, the mechanical strength of the alloy degrades significantly above 100 degrees C. Another common alloy known as HMP comprises 93.5% lead, 1.5% silver, and a tin balance. This alloy provides good results for operating temperatures over 200 degrees C., has a solidus temperature of 296 degrees C. and has a liquidus temperature of 301 degrees C. This solder is moderately compatible with tin and lead plating provided that the solder joint volume is large such as in wave solder applications. Another binary eutectic alloy is Sn96, having 96.5% tin and a silver balance. The melting point is 221 degrees C., it is mechanically stronger than HMP or Sn63, and retains its strength nearly until the melting point. The wettability is excellent, and the alloy permits mass reflow of plastic surface mount parts without damage. The disadvantages of this alloy are brittleness, particularly at low temperatures, and an affinity for copper. The brittleness results in low cycle solder joint fatigue with repeated temperature cycling when the thermal coefficients of expansion of the PWB substrate and the component are mismatched. For components with high copper content in the contacts, migration of tin into the copper contact results in electrical and mechanical failure after short time periods when the exposure exceeds 150 degrees C.
As described above, tin and lead are incompatible for high temperature solder applications. High lead solders have high melting points and low mechanical strength. For automated plastic surface mount assembly, exposure of such components to high temperatures destroy the components. If the solder is the only mechanism for attaching electronic components to the PWB, the integrity of the PWB will be compromised at high temperatures. This limitation is particularly acute in applications having shock or vibration forces. High tin solders are too brittle when the thermal coefficients of expansion are mismatched, and tin-copper intermetallic growth causes premature joint failure at high temperatures. Although gold or palladium can improve the operating characteristics, such materials also create brittle intermetallics over time and generate problems in manufacture and assembly of the electronics packages.